Optical disk initialization method and apparatus

ABSTRACT

A method and apparatus for initializing an optical recording medium comprising a phase change material using a series of flashes of light. A series of low-power, high duty cycle flashes of light are produced to initialize the media to a crystalline form, onto which data may be recorded. In accordance with another aspect of this invention, a spiral flash bulb with centrally located connectors comprises the flash illumination source for initializing the optical recording media.

This is a continuation of U.S. Ser. No. 09/746,748, filed Dec. 22, 2000now U.S. Pat. No. 6,665,245.

BACKGROUND OF THE INVENTION

The invention relates to processes of initializing disks for opticaldata storage.

DESCRIPTION OF THE RELATED ART

In a phase transformation optical recording medium, data is generallyrecorded by transforming the phase of each recording bit of therecording layer from a crystalline state to an amorphous state. Ingeneral, since recording is conducted using phase transition between thetwo states, the phase of the recording thin film should be converted tothe crystal phase in advance. This operation is called initialization,signifying the transformation of the whole recording region on therecording layer to the crystalline state prior to the recording of data.

One method of initialization, typically referred to as serialprocessing, uses a laser beam for initialization. With this method, alaser light spot of from several tens to several hundreds of μm indiameter is formed with the output far greater than that of a typicallaser diode for recording and reproduction. By irradiating and rotatingthe media at a constant speed, many tracks can be crystallized in asingle operation. This method has the advantage that the medium isheated in a small area at a time, reducing thermal load and theconsequent risk of material cracking. However, because only a small areais illuminated at any one time, this method has the disadvantage thatinitialization of an entire disk takes a long time.

To overcome some of the difficulties associated with serial methods ofoptical initialization, flash methods of initialization have beendeveloped. By irradiating the media with a flash lamp, the recordinglayer across the entire surface of the optical recording medium can beinitialized simultaneously. With initialization by flash irradiation,the initialization itself may be completed in a period of only severalmilliseconds. One problem with this method, however, is that therecording medium is heated at once, producing thermal induced internalstresses that increase the likelihood of thermal damage to both thesubstrate and the coating, such as warping, cracking, and other forms ofdistortion. In U.S. Pat. No. 5,684,778 to Yamada et al., this problem isaddressed by controlling the duration of the flash. As the flashduration decreases, however, flash intensity must be increased tosuccessfully produce the desired phase change in the recording material.This requires a resulting increase in power supply capacity and expense,and also increases the time required to charge the flash capacitor,thereby slowing any process of sequential initialization of disks duringthe disk manufacturing process. Other attempted solutions include theuse of a complex thermal heat management platten.

Another problem with the flash initialization method is that theuniformity of initialization over the disk surface is difficult toproduce. Spatial non-uniformity in the light intensity of the flashproduces non-uniform initialization. In addition, the structure of thedisk results in a reflectivity profile which varies significantlydepending on wavelength and angle of incidence. A single flash,therefore, will typically produce a non-uniform initialization that hasheretofore been unacceptable for the production of optical disksintended for high density data storage. Making the illumination moreuniform with, for example, an optical tunnel is very inefficient andrequires higher input flash power with the associated problems outlinedabove.

SUMMARY OF THE INVENTION

The present invention relates to a method of initializing an opticalrecording media. In preferred embodiments, this process will comprisethe exposure of the media to at least two flashes of light. Theseflashes are preferably less than about five hundred micro-seconds induration, and are preferably repeated at a rate of at least one flashper second. In other embodiments, the flashes may be repeated five ormore times, and for 100 to 200 microseconds per flash. These repetitionsmay occur as frequently as 10 to 50 flashes per second. For example,five 150 microsecond flashes repeated at a rate of 30 Hz.

In accordance with another aspect of this invention, an apparatus isprovided for initializing optical recording media. This apparatuspreferably comprises one or more flashbulbs, a support for the media toallow adjustable proximaty to the flashbulbs which may be rotatable (foroff-line initialization) or a support for the flash head that allowsadjustable proximity to the media for in-line initialization, acapacitor, a charging circuit, a switching circuit coupling thecapacitor to the charging circuit. Additionally, the control circuit ispreferably configured to repetitively charge the capacitor with thecharging circuit and to discharge the capacitor through one or more ofthe flashbulbs at a rate of at least about one discharging and chargingcycle per second. In preferred embodiments, the flashbulbs may be aspiral shape. Preferably, the spiral begins with a connector proximateto the center of the spiral, and ends with a connector proximate to thecenter of the spiral as well.

In accordance with another aspect of this invention, a method isprovided for initializing optical recording media comprising exposing alayer of material of the media to a series of two or more low energy,high duty-cycle flashes of light. In further embodiments, this methodmay comprise a series of two or more low energy, high duty-cycle flashesof light wherein each of the flashes has sufficient intensity andduration to only partially crystallize the layer of material.

In another preferred embodiment of this invention, an apparatus forinitializing an optical recording medium is provided comprising aflashbulb having a substantially spiral shape, wherein the spiral beginsand terminates proximate to the geometric center of the spiral, orterminates at the outside diameter, and wherein the spiral has anoverall diameter which is approximately equal to or larger than thediameter of said recording media. A support for the media aligned withthe flashbulb.

In yet another preferred embodiment of this invention provides for anoptical recording media made a process comprising the steps ofpositioning the media proximate to a flashbulb and exposing the media toa series of two or more flashes from the flashbulb, wherein each flashsuccessively increases a crystallization state of a layer of material onthe media, or the fraction of the crystallized area on the disc, andwherein the layer of material is substantially completely crystallizedby the series of flashes within approximately five seconds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of an initialization process in accordance withone embodiment of the invention.

FIG. 2 is a block diagram of an apparatus for initializing opticalrecording media;

FIG. 3A is a plan view of a prior-art spiral shaped flashbulb havingconnectors near the edge of the spiral;

FIG. 3B is a plan view of a spiral shaped flash-bulb having connectorsnear the center of the spiral;

FIG. 3C is a schematic of another spiral shaped flash-bulb comprisingtwo separate tubes, and having connectors near the center of the spiral;

FIG. 4 is a measurement of reflectivity along two tracks of an opticalrecording media, after the media has been initialized in accordance withone embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the invention will now be described with reference to theaccompanying Figures, wherein like numerals refer to like elementsthroughout. The terminology used in the description presented herein isnot intended to be interpreted in any limited or restrictive manner,simply because it is being utilized in conjunction with a detaileddescription of certain specific embodiments of the invention.Furthermore, embodiments of the invention may include several novelfeatures, no single one of which is solely responsible for its desirableattributes or which is essential to practicing the inventions hereindescribed.

The present invention avoids many of the disadvantages of prior artinitialization methods with the use of at least two separate andindependent approaches. One approach advantageously initializes themedia with a series of multiple short duration flashes rather than asingle flash. Preferably, the pulses of the series each have an energytoo low to completely crystallize the media, and are applied at a highduty cycle such that complete crystallization is performed within aboutfive seconds.

In general one needs to supply a pulse that is longer than thecrystallization time of the phase change material, typically greaterthan 90 nano-seconds, but less than a millisecond and preferably on theorder of 100 micro-seconds. Above about one millisecond, the medium willeither not initialize at all or will crack for any flash intensitydelivered. Below a millisecond, one can find a combination of intensityand pulse duration and number of pulses that will intialize a givenphase change medium. One way of understanding this is in terms ofcontrolling surface energy. If too much light energy is delivered to themedium, any energy in excess of that required to crystallize (initalize)the medium goes into surface heating, resulting in cracks in thecoating. Therefore, controlling the surface energy to just that requiredfor initialization prevents the formation of cracks.

One may also understand the success of this short multi-pulse flashmethod in terms of thermal dissipation and relaxation. The reason thatthe laser serial intialization works is that the local heating is quickas the small laser spot moves by, allowing equally quick dissipation ofthe heat and the coating to “relax” before thermal damage to the coatingcan occur. In our short multi-pulse flash method, the entire surface isexposed at once but the short duration of the pulse allows the heat toquickly dissipate, thereby preventing damage to either the coating orthe substrate.

In some optical recording schemes it is desired to write higherreflectance crystallized marks into a darker substantially amorphousmedium. The flash method for this case is stopped short of fullinitialization, thereby leaving the medium substantially amorphous butin a so-called primed condition, in which very small seeds forcrystallization growth have been formed. With the medium in this state,one may write marks that are smaller than would be possible otherwise,and with less energy. The flash initialization method is similarlyadvantageous for priming medium as well therefore. It can be seen thatwith our multi-pulse method the priming can be controlled easily,compared to a single flash method or to a scanned laser method.

In addition to this multiple flash technique, several features areprovided to increase the uniformity of the initialization. Theseinclude, for example novel light sources and reflective components thatenhance the spatial uniformity of flash intensity.

FIG. 1 is a flow chart that describes one embodiment of a process ofinitializing an optical recording media. The first step 20 involvesexposing the media to a first flash of light. As described above, theflash at step 20 may comprise a relatively low power, short durationpulse of light. Flash duration is preferably controlled so as to be lessthan about 1 millisecond, with approximately 150 microseconds havingbeen found suitable in one embodiment. The first flash at step 20 isfollowed by a second flash at block 22 after a delay period. To performthe initialization process quickly, it is advantageous for the delay tobe less than about one second. In preferred embodiments, this delay isless than about 100 milliseconds. In one specific embodiment, the delayis about 33 milliseconds, corresponding to a flash duty cycle ofapproximately 30 Hz. The second flash 22 will generally be approximatelyequal to the first in intensity and duration, although variations inthese flash parameters are possible and may be desired in someapplications.

At step 24 a determination is made as to whether or not further flashesof light are required. In preferred embodiments, the number of flashesmay be predetermined, and the only factor in the decision at block 24 iswhether or not the flash count has reached the desired number. It willbe appreciated that there is large latitude afforded by the flashinitialization method and apparatus in that the medium typically cannotbe over-initialized. The flash output and pulse length and number ofpulses can be determined for a given medium beforehand, and then anextra number of pulses can be supplied in production to allow formanufacturing variances in the coating or medium. In this way theinitialization is saturated and any over-pulsing does no damage as longas the initial parameters are set correctly with a sample disc.

The number of pulses, duration of the pulses, and intensity of thepulses (and /or proximity between the flash and the media) needs to bedetermined for different phase change stack recipes, whether thedifferences are in the design coating thickness or different phasechange materials altogether (AIST vs. GST for example), or manufacturingvariances. In general, however, the adjustment required is minor.

In some alternative embodiments, however, a monitoring system may bequeried as to the condition of the media during step 24. This monitoringsystem may be optical in nature. For example, it may directly scan thesurface of the media using a laser, or the equivalent, or the system mayuse machine vision techniques to ascertain whether or not theinitialization process is complete.

If a further flash of light is determined to be required during step 24,the system will return to step 24 and produce another flash.Subsequently, the process will continue to loop back to step 22 until itis determined that another flash of light is no longer required. Whenthe flash is no longer required, the process ends at block 26.

FIG. 2 shows a block diagram of one embodiment of an apparatus forinitializing optical recording media. This apparatus preferablycomprises a support 30, a flashbulb 32, control circuitry 34, capacitor36, switch 38, charging circuitry 40, and power supply 42. The support30 supports the media 44 proximate to the flashbulb 32, and positionsthe media 44 to a preferred location during the initialization process.The media 44 may, for example, be mounted about one inch from theflashbulb 32. The power supply 42 is connected electrically to thecharging circuitry 40, which regulates the charging current and voltagewhich is supplied to the capacitor 36. The capacitor 36 is in serieswith the charging circuitry 40, as well as the switch 38. The switch 38functions to discharge the capacitor 36 through the flash bulb 32,thereby creating a flash of light. To control capacitor charging andswitching of the capacitor, the control circuitry 34 is connectedelectrically to the switch 38 and the charging circuitry 40.

The control circuitry 34 is advantageously configured to control thepattern, duration, and intensity of the flashes of light. As describedabove, the control circuitry 34 may be configured to perform at leastone capacitor charge and discharge cycle per second so as to produce atleast approximately one flash per second. The control circuit 34 mayalso be coupled to the support 30 so as to control and alter theorientation of the media 44 during the initialization process. In oneembodiment, the support 30 is rotated up to 90 degrees between eachflash. In other embodiments, the support is rotated up to 90 degreesevery two flashes. The rotation may be continuous or stepwise.

In some cases, the material to be initialized is manufactured as acontinuous thin sheet or web. In this case, the flashbulb 32 can bepositioned over the web and flashed continuously at, for example, the 30Hz repetition rate described above. In this embodiment, the web can bescrolled beneath the flashbulb 32 at a rate that exposes each portion ofthe media to the desired number of flashes as it passes under theflashbulb 32.

There are a variety of flashbulbs, fixtures, and charging/dischargingapparatus currently on the market that are suitable for use with thepresent invention in the system illustrated in FIG. 2. In contrast withprior art flash initialization methods, each flash has a relatively lowintensity and short duration. Thus, only a relatively small amount ofenergy need be stored in the capacitor between flashes. This allows fora very short charging time, which in turn allows for a high flash dutycycle, and very fast initialization. Five flashes at 30 Hz, for example,will initialize the media in less than 170 milliseconds and may berepeated on subsequent disks essentially immediately with a singleinitialization apparatus, as compared to several seconds or even minutesper disk with a single apparatus in prior art techniques.

On a micro-scale, the uniformity from the flash illumination is againsuperior to the micro-scale uniformity obtained with a scanned laserspot for two reasons. First, the light from the flash is incoherent sothere is none of the interference speckle patterning inherent to acoherent laser source. Second, the flash illumination is whole-field, sothere are none of the remnant scanning tracks from the laser (even withcareful overlap of the scanned laser tracks, there remain substantialnon-uniformities on this micro-scale).

As mentioned above, producing a uniform field of illumination with theflash of light is highly desirable, while maintaining optical efficiencyto avoid higher power requirements, and thereby longer charge times, forthe flash. This is accomplished through careful design of the flash tubeitself to greatly enhance the uniformity of the initialization process.FIG. 3A shows a prior art arrangement of a spiral shaped flash bulb.This spiral begins with a first connector 56 proximate to an outer edgeof the flash bulb. The tube 58 then winds in a largely circular fashiontoward the center of the spiral. Upon reaching a specific innerdiameter, the tube 58 is then brought back out of the spiral, typicallybeneath the exposure surface of the flash bulb, to place the finalconnector 60 proximate to the first connector 56. In this prior artflash bulb, asymmetric intensity patterns are generated near the outsideedge of the flashbulb corresponding to the location of the connectors56, 60 on the tube. Because the recordable media extends radially fromthe center to the edge of the disc, intensity fluctuations near the edgeof the media jeopardize the uniformity of the initialization processnear those locations.

FIG. 3B shows a flashbulb arranged in accordance with one embodiment ofthe invention having connectors 62, 64 proximate to the center of theflash bulb. In use, the intensity fluctuations which may be caused bythe connectors 62, 64, or the variations of the tube proximate to theconnectors 62, 64, do not affect the initialization process as muchbecause the center of the optical disc typically comprises a hole oropening in the center, rather than optically recordable media.

It has also been noted that intensity fluctuations are caused by thespatial separation between loops of the spiral. FIG. 3C shows a flashtube arranged in accordance with another embodiment of the inventionwhich serves to reduce this source of non-uniformity. In thisembodiment, the flash bulb comprises two independent tubes wound in aconcentric spiral configuration. Each tube has a first and secondconnector, both of which are proximate to the center of the spiral forreasons disclosed above. Additional preferred embodiments may compriseflash tubes of small diameter, and wound in more frequent spirals.Further preferred embodiments comprise different diameter tube woundaround a common center. Additionally, the number of element thatcomprise the flash bulb may be numerous, for example, three filamentsall with different radii tubes wound around a common center, terminatingproximate to the center of the spiral.

Additional uniformity is obtained because the medium itself is close tothe flash, and so it serves to partially reflect light back to a flashreflector 52 that may be provided on the opposite side of the flashbulb, and thus forming part of a multiple reflection mirror box thatincrease uniformity by multiple reflections between the disk and theflash lamp. The reflector 52 may comprise a stippled surface to increasethe diffusivity of the reflector during operation, Additionally, aprotective glass or quartz plate 50 on the flash head may be made with asmall diffusivity, such that as the light passes through the plate 50,the diffusion contributes to the uniformity of the incident light. Inother preferred embodiments of this plate 50, the plate 50 comprises acoating that has an optimum reflection/transmission profile for variousregions of the plate 50 matching that of the media and for differentwavelengths to optimize the absorption by the media duringinitialization. And in yet another embodiment of this plate, a locallyvaried neutral density filter is formed to absorb light more strongly inthe higher intensity part of the flash output and less strongly in thearea of the tube with lower output. Such a filter is easily made inexact calibration to the tube by using the tube itself to make it. Aphotographic plate or other photsensitive material is place on the tubeand exposed and then developed or otherwise “fixed” to prevent furtherchanges upon subsequent exposure. This plate is then returned to theflash head and aligned with it just as before, thereby making the lightoutput uniform. However, efficiency suffers in this scheme.

Finally, a proximity mask can be placed against the medium, in which themask has prefabricated patterns that upon exposure with the flash areimprinted onto the optical medium. The pattern can include micro-scaleinformation such as formatting and tracking information, or moremacro-scale information such as company logos, identification numbers,designs, or security codes including bar codes.

EXAMPLE

A DVD-RW disk comprising a layer of phase change recordable material ofAIST in a substantially amorphous state was exposed to a series of five150 microsecond flashes of light at a duty cycle of 30 Hz (i.e. about 33milliseconds between flashes) using a Xenon Corp. RC-747 flashlamp withthe XL-1890 spiral tube mounted within. About 207 Joules of storedelectrical energy was discharged through the lamp fixture with eachflash. A highly uniform crystallized state was observed following thetreatment, as illustrated in FIG. 4.

FIG. 4 is a graph of measured reflectivity along two different tracksaround the disk after this initialization treatment. Each trace wascollected using an Infinium scope in peak detect mode. The measured SDRwas 0.93%. The smoother trace and lower SDR value indicates that themedium is much more uniform on a micro-scale, which again results fromthe whole-field illumination form the large area flash, as compared tothe serial scanned spot of a laser initializer.

The foregoing description details certain embodiments of the invention.It will be appreciated, however, that no matter how detailed theforegoing appears in text, the invention can be practiced in many ways.As is also stated above, it should be noted that the use of particularterminology when describing certain features or aspects of the inventionshould not be taken to imply that the terminology is being re-definedherein to be restricted to including any specific characteristics of thefeatures or aspects of the invention with which that terminology isassociated. The scope of the invention should therefore be construed inaccordance with the appended claims and any equivalents thereof.

1. A method of initializing an optical recording medium comprising:exposing said medium to a first flash of light from a light source, saidflash of light having a duration greater than one micro-second and lessthan about one millisecond; and repeating said exposure one or moreadditional times at a rate of at least about one flash per second. 2.The method of claim 1 comprising exposing said media to two or more 100to 200 micro-second flashes.
 3. The method of claim 2, furthercomprising generating said flashes at a repetition rate of approximately10 to 50 flashes per second.
 4. The method of claim 1, comprisingexposing said media to five approximately 150 micro-second flashes. 5.The method of claim 1, wherein the process is stopped beforeinitialization is complete to prime the surface for writing crystalizedmarks on amorphous medium.
 6. The method of claim 1 further comprisingthe step of rotating said media during said exposing.
 7. An apparatusfor initializing an optical recording media comprising: one or moreflashbulbs; a support configured to hold said media in approximatealignment with said one or more flashbulbs; and a control circuitconfigured to flash said one or more flashbulbs, at a rate of at leastabout one flash per second, wherein said flash has a duration greaterthan one micro-second and less than about one millisecond and said mediais exposed to two or more flashes.
 8. The apparatus of claim 7, whereinsaid support is rotatable.
 9. The apparatus of claim 7, wherein said oneor more flashbulbs is in a spiral configuration having ends locatedproximate to the center of the spiral.
 10. The apparatus of claim 7,further comprising a transparent or semi-transparent plate between saidone or more flashbulbs and said media.
 11. The apparatus of claim 10,wherein said plate is diffusing.
 12. The apparatus of claim 10, whereinsaid plate comprises a variable neutral density filter formed byexposure of a photosensitive material to said flash.
 13. The apparatusof claim 7, wherein an outside diameter of one or more flashbulbs islarger than an outside diameter of said media in order to assure properillumination uniformity at outside parts of said media.
 14. Theapparatus of claim 7, wherein said control circuit is further configuredto expose said media to two or more 100 to 200 micro-second flashes. 15.The apparatus according to claim 7, wherein said control signal isfurther configured to generate said flashes at a repetition rate ofapproximately 10 to 50 flashes per second.
 16. The apparatus of claim 7,wherein said control circuit is further configured to expose said mediato five approximately 150 micro-second flashes produced at a repetitionrate of approximately 30 flashes per second.
 17. The apparatus of claim7, wherein said control circuit is further configured to stopinitialization before initialization is complete to prime a surface ofsaid media for writing crystalized marks on amorphous medium.
 18. Amethod of initializing an optical recording media comprising exposing alayer of material on said media to a series of two or more low energy,high duty-cycle flashes of light.
 19. The method of claim 18, whereineach of said flashes has sufficient intensity and duration to onlypartially crystallize said layer of material.
 20. The method of claim19, wherein said duty cycle is high enough to substantially completelycrystallize said layer of material within approximately five seconds.21. The method of claim 19, wherein said duration is greater than onemicro-second and less than about one millisecond.
 22. The method ofclaim 19, wherein said duty cycle is more than about 10 flashes persecond.
 23. The method according to claim 1, comprising exposing saidmedia to flashes produced at a repetition rate of approximately 30flashes per second.
 24. The apparatus of claim 10, wherein said plate iswavelength-selective.
 25. The apparatus of claim 10, wherein said platecomprises a variable neutral density filter.